There has been sought widespread use of renewable power supplies that are capable of reducing the emission of greenhouse gases while in operation in order to achieve a sustainable society. In particular, solar cells are a renewable power supply that is most promising at present. One problem with the solar cells, however, is that their power output varies depending on the weather, or stated otherwise, the amount of electric energy generated thereby is unstable. Accordingly, if the number of solar power generation facilities that are linked to a power distribution system increases, then their unstable power generation capability is expected to adversely affect the power distribution system, causing problems referred to as “distributed voltage destabilization” and “supply-demand imbalance”. There has been a concern in the art about these problems as they are liable to prevent renewable power supplies from finding widespread use, and demand has been increasing for finding a way to resolve these problems. The problems “distributed voltage destabilization” and “supply-demand imbalance”, and solutions that have heretofore been proposed to solve these problems will be described below.
The problem “distributed voltage destabilization” is that the voltage (about 100 V) which is supplied to various electric power users including private houses, offices, autonomous facilities, etc. varies beyond an appropriate range due to the unstable amount of electric power generated by renewable power supplies. A voltage of about 6600 V sent from a power-distributing substation in a power distribution system is converted by the lowest-voltage transformer (hereinafter referred to as “terminal transformer”) in the power distribution system into a voltage of about 100 V, which is supplied to electric power users. At this time, as current flows through power distribution lines having a limited resistance, the distribution voltage varies depending on the demand (electric power consumption). So far, demand has differed between daytime and nighttime, resulting in variations in the distribution voltage. However, the distribution voltage has been kept within an appropriate range of 101V±6V because changes in demand have been sufficiently small.
If the number of renewable power supplies that are linked to the power distribution system increases, then since the instability of the renewable power supplies is added to changes in demand, the distribution voltage tends to vary greatly beyond the appropriate range.
One existing technology for solving the problem “distributed voltage destabilization” is a process which uses a shutoff function that an inverter has. The inverter is a device for converting electric power generated by a renewable power supply into electric power that is compatible with a power distribution system. The inverter monitors the distribution voltage. When the distribution voltage approaches the upper limit of the appropriate range, the inverter breaks the linkage of the renewable power supply to the power distribution system, preventing the distribution voltage from rising.
The above process, however, leads to a reduction in the availability ratio of renewable power supplies as the number of renewable power supplies linked to the power distribution system grows, tending to waste the power generating capability of the renewable power supplies.
The problem “supply-demand imbalance” is that the demand and supply of electric power in an overall electric power system are brought out of balance as the number of renewable power supplies that are linked to the power distribution system increases. The overall electric power system needs to keep demand and supply of electric power in agreement with each other. Heretofore, the demand and supply of electric power have been kept in balance because the electric power company adjusts the amount of generated electric power depending on changes in the demand. Specifically, some electric generating stations that have an electric power demand-supply adjusting capability, such as thermal power stations, have played the adjusting role. An increase in the number of renewable power supplies that are linked to a power distribution system means an increase in the number of electric generating facilities which do not have an electric power demand-supply adjusting capability and whose generated electric power is unstable. Therefore, as the number of renewable power supplies that are linked to a power distribution system increases, the electric power demand-supply adjusting capability of the power distribution system weakens, making the frequency unstable and tending to cause large-scale blackout in a worse-case scenario.
An existing technology for solving the problem “supply-demand imbalance” is a process of increasing the number of thermal power stations that operate at a low availability ratio to increase the electric power demand-supply adjusting capability of an entire electric power system.
However, when thermal power stations operate at a low availability ratio, the cost of generating electric power increases because their power generating efficiency is low. Furthermore, increasing the number of thermal power stations contradicts to the purpose of reducing the emission of greenhouse gases by increasing the number of renewable power supplies.
As described above, no drastic solutions to the problems “distributed voltage destabilization” and “supply-demand imbalance” have been found in the art. For drastically solving the above problems, it is necessary to reduce the instability itself of renewable power supplies. As a way of reducing the instability of renewable power supplies, Patent documents 1 through 4 have proposed technologies for connecting a storage battery, which serves as a buffer for managing the instability of the amount of generated electric power, to the output of a renewable power supply.
In recent years, widespread use of storage batteries for use on vehicles has reduced the cost of storage batteries, and a process of linking storage batteries for use on vehicles to a power distribution system has been proposed (see Non-patent document 2), making it practical to install large-capacity storage batteries in private houses, etc. However, since storage batteries are still expensive, it is essential to lower the cost of storage batteries so that their usage can become widespread. Patent documents 1 through 4 do not discuss anything significant as regards lowering the cost of storage batteries.
For lowering the cost of storage batteries, it is necessary not only to reduce the manufacturing cost of storage batteries, but also to reduce the capacity of storage batteries used by respective electric power users but to increase the product life cycles of storage batteries (longer product life cycles). A reduction in the capacity of storage batteries realizes a reduction in the installation cost of storage batteries, and the longer product life cycles of storage battery realizes a reduction in the cost required for the maintenance of electric generating facilities which use storage batteries and renewable power supplies. Efforts to achieve the “longer product life cycles” or “increased product life cycles” of storage batteries according to the present description mean efforts to prevent a reduction in the product life cycles of storage batteries owing to the operating process, but not efforts to prolong the product life cycles of storage batteries based on improved battery materials.
Processes that are presently proposed for reducing the capacity of storage batteries and making the product life cycles of storage batteries longer will be described below.
According to one process of reducing the capacity of storage batteries, a plurality of storage batteries are installed in a centralized area (centralized installation), rather than being installed at respective electric power users with power generation facilities (distributed installation). If storage batteries are installed in distributed locations, then the electric power users need to install storage batteries which have a sufficiently large capacity for individual charging and discharging demands thereof. Therefore, the total capacity of all the storage batteries belonging to the electric power users becomes large. If storage batteries are installed in a centralized area, then since the amounts of electric power to be charged into and discharged from the respective storage batteries even out because the demand of electric power from the electric power users and the supply of electric power to the electric power users cancel each other out, it is possible to reduce the total capacity of all the storage batteries and, as a result, to reduce the capacity of the storage battery belonging to each of the electric power users (Non-patent document 3).
However, the centralized installation of storage batteries (e.g., in each power-distributing substation) cannot solve the above problem “distributed voltage destabilization” because electric power generated by renewable power supplies is supplied via a power distribution system that charges the storage batteries and hence a number of electric generating facilities whose generated electric power is unstable are linked to the power distribution system.
The distributed installation of storage batteries reduces the effect of power generation instability on a power distribution system by managing the power generation instability with the storage batteries that are installed near renewable power supplies, though the distributed installation makes the total capacity of the storage batteries large. Consequently, the distributed installation of storage batteries is effective to assist in solving the problems “distributed voltage destabilization” and “supply-demand imbalance” of a power distribution system. Therefore, a technology for installing storage batteries in distributed locations, while at the same time reducing the total capacity of the storage batteries, is sought in environments where a number of electric generating facilities are linked to a power distribution system.
According to one process of prolonging the product life cycles of a storage battery the number of charging and discharging cycles is reduced. Generally, many types of storage batteries typified by a lithium ion battery have their capacity lowered as they are repeatedly charged and discharged. In order to prevent the product life cycles of a storage battery from being shortened, ro therefore, it is desirable for the storage battery not to be charged and discharged frequently repeatedly. In addition, inasmuch as it is known that the deterioration of a storage battery is accelerated if it is used in a high SOC (State Of Charge: the ratio of a charged amount of electric power to the capacity of the storage battery) range, it is desirable for the storage battery not to be used in a range of high SOCs (hereinafter referred to as “high SOC range”). Moreover, if a storage battery is kept in storage after it has been charged to a high SOC range, then the charged energy is wasted as the storage battery is self-discharged to a large degree in the short run, and the product life cycles of the storage battery is shortened as the storage battery is deteriorated in the long run.
For example, Patent document 5 and Patent document 6 disclose a technology wherein two storage batteries are provided for use on an electric vehicle, and while one of the storage batteries is being discharged, the other storage battery is charged, with their charging and discharging processes alternating with each other when necessary thereby reducing the number of charging and discharging cycles.
According to the technology disclosed in Patent document 5 and Patent document 6, however, the amount of electric power charged in a storage battery, which is to be switched from a charging cycle to a discharging cycle, may not necessarily have reached a sufficient level, and the amount of electric power stored in a storage battery, which is to be switched from a discharging cycle to a charging cycle, may not necessarily have reached a minimum level. Therefore, although the number of charging and discharging cycles can be made smaller than with a system not based on the technology disclosed in Patent document 5 and Patent document 6, the disclosed technology fails to sufficiently reduce the number of charging and discharging cycles of storage batteries.
Patent document 7 discloses a technology for not keeping a storage battery on a vehicle in storage after it has been charged to a high SOC range, by failing to charge the storage battery if it is expected that the vehicle will not be used for a long period of time.
Storage batteries on vehicles do not need to be operated if the vehicle is at rest. However, storage batteries used by electric power users including private houses, offices, autonomous facilities, etc. may store electric power generated by renewable power supplies and discharge stored electric power in response to a request from energy systems connected thereto, even in the absence of electric power users. Therefore, a need has arisen for a technology for operating a storage battery while at the same time not using a high SOC range as much as possible.
Generally, a storage battery made up of a plurality of storage cells or modules for increased battery capacity and output voltage suffers individual cell characteristic variations. Patent document 8 and Patent document 9 disclose a technology for reducing characteristic differences between cells at the time they are charged and discharged, thereby extending the product life cycles of the overall storage battery. However, the technology disclosed in Patent document 8 and Patent document 9 is unable to reduce the number of charging and discharging cycles of the individual cells though it can minimize a reduction in the product life cycles of the storage battery that results from the individual cell characteristic variations.
As described above, it is desirable to use storage batteries and install the storage batteries in distributed locations in order to lessen the problems “distributed voltage destabilization” and “supply-demand imbalance” which are caused by renewable power supplies, particularly solar cells, linked to a power distribution system.
However, the distributed installation of storage batteries tends to generate an increase in the capacity and cost of the storage batteries. As described above, in order to lower the cost of storage batteries, it is important to minimize a reduction of the capacity thereof while at the same time extending the product life cycles thereof.